What are good free PBR material libraries for a beginner?
You should rather watch vidoes and try to create your own library so that u can learn as well. I would suggest you to watch videso from blender guru (https://youtu.be/V3wghbZ-Vh4) and remington graphics (https://youtu.be/OulP6f-aF-c). You can learn from these videos and create your own shaders. If not then i think remington has provided the shader pack for free as well.